To analyze the initial performance of rubber products or the performance of aged rubber products, the following method is widely used: dumbbell specimens cut out of the products are broken to measure the elongation at break (%) and tensile strength (MPa) using a tensile tester according to JIS-K 6251 “Rubber, vulcanized or thermoplastic-Determination of tensile stress-strain properties”, and the breaking energy (the area below a stress-elongation curve) is then calculated from the resultant values.
A typical tensile test requires the preparation of a specimen of 35 to 120 mm in width, 6 to 25 mm in length, and 2 mm in thickness without any cracks and the like to provide accurate test results. This requires cutting a sheet-shaped specimen out of a product and then cutting the specimen into a dumbbell shape, which is greatly time-consuming. Moreover, though the initial performance can be analyzed according to this method, it is impossible in practice to perform the tensile test on deteriorated samples because many of such deteriorated samples have cracks and thus are difficult to prepare to obtain evaluable specimens. Thus, determination of the breaking energy in accordance with JIS-K 6251 has various disadvantages.
Meanwhile, since high breaking energy is generally considered to be important for the securement of good deterioration resistance, in particular crack resistance, rubber compositions excellent in crack resistance have been developed on the basis of their breaking energy values. However, the breaking energy varies depending on the formulation or vulcanizing conditions though it is an important parameter. Therefore, it is considered to be further important to determine the breaking energy of aged rubber materials in accordance with JIS-K 6257 “Rubber, vulcanized or thermoplastic—Determination of heat ageing properties” for evaluation of crack resistance.
However, the tensile test cannot be performed on deteriorated samples due to the difficulty in preparing evaluable specimens therefrom as described above. Therefore, it is also difficult to develop a rubber composition excellent in crack resistance referring to the breaking energy as an indicator.